NL1017412C2 - Method for protecting surfaces against biological fouling. - Google Patents

Description

Method for protecting surfaces against biological fouling.

The invention relates to a method for protecting surfaces against biological fouling by applying a potential that fluctuates over time.

The electrochemical protection of ship hulls against biological fouling by applying varying potentials is described by Sandrock and Scharf in a poster entitled "Electrochemical method of protection against marine growth adhering to ship hulls" presented at the Marine Biofouling symposium, held at 7 July 9, 1999 at the University of Plymouth, Plymouth, England. The method described herein uses a special three-layer coating system that consists of an insulating primer layer, a thin titanium layer, and an electrically conductive polymer layer / paint system that must protect the titanium layer against seawater. By alternately applying a positive and negative potential to the titanium layer, an acidic and alkaline environment is alternately created on the surface of the three-layer system. However, the application of the described three-layer coatings to sea-going vessels is particularly expensive and, moreover, such coatings are expected to be very delicate.

It is known to use cathodic protection to protect metal-containing surfaces that come into contact with water against oxidative corrosion. This method is based on applying a constant negative potential. A description of how cathodic protection works can be found in "Cathodic and Anodic Protection", chapter 10 in "Corrosion", Volume 2, edited by L.L. Shreir, R.A. Jarman, G.T. Burstein, 3rd edition (1994), 25 published by Butterworth-Heinemann Ltd, Oxford, UK. In contrast to the described method of active cathodic protection, the method according to the present invention uses a fluctuating potential.

Organisms will deposit on almost all materials that are placed in water. This starts with the formation of a very thin biofilm by micro-organisms. At a later stage, larger organisms will nest on the material, creating macro-fouling, here referred to as "fouling".

Fouling on ships, structures and in installations that are in contact with (sea) water is an important problem. For example, growth of smallpox or 10174> i 2 mussels can lead to a strong increase in the resistance of ships in the water, to blockages in pipe systems, to microbiological corrosion, to a deposit attack, to erosion corrosion or to a reduction in heat transfer. . Continuous research is therefore being carried out to increase the efficiency of antifouling methods.

At this time, fouling in open systems is often controlled by applying an anti-fouling coating, an anti-fouling paint. In most cases, the anti-fouling effect is based on the slow dissolution of a toxic component in these coatings.

However, the use of an anti-fouling paint has a number of disadvantages.

These include

harmful effects on the environment; the need to periodically replace the paint system, which slowly loses its anti-fouling effect; - a coating must be applied for antifouling, also when this is not necessary or even undesirable from a corrosion point of view, with a view to heat transfer or construction-wise.

In addition to the use of coatings, open cooling water systems also offer the option of dosing biocides. Commonly used methods are dissolving copper at the water inlets 20 or producing chlorine or hypochloride, which is entrained by the cooling system and has an anti-fouling effect there. However, also for these systems, environmentally harmful components are introduced into the seawater.

The invention is aimed at overcoming the above drawbacks and relates to a method for protecting against biological fouling of surfaces (S) that are in contact with or come into contact with a water-containing medium (M) wherein 1) S is electrically conductive and 2 ) is applied to S such a potential (P) which fluctuates over time that it impedes the growth of organisms that live in M and / or reproduce therein and that tend to form deposits on S, characterized in that P does not assume values higher than the corrosion potential of S in M and the average value of P is lower than the same corrosion potential. The corrosion potential is defined as the potential of a corrosive surface in an electrolyte relative to a reference electrode, as defined in "Principles and Prevention of Corrosion" by D.A. Jones, second edition, Prentice-Hall, Upper Saddle River, NJ 07458. In this description, the specified values 5 apply to the potential (P) relative to a saturated calomel electrode (SCE).

By "biological fouling" is also meant deposits due to the presence of (micro) organisms that are or can be present in seawater, brackish water and freshwater or water-containing media or systems.

Compared with the method as described by Sandrock and Scharf, the method according to the invention has the important advantage that it is much simpler, since the application of a special three-layer coating is superfluous. The fluctuating potential can be applied directly to, for example, the ship's hull or the pipes of a system through which water is displaced. The latter is possible because, unlike Sandrock and Scharf, the present method does not use potentials that are higher than the corrosion potential. Therefore, no precautionary measures need to be taken to prevent accelerated corrosion from occurring on corrosion-sensitive components, such as a ship's hull.

Although it is not entirely clear how the method according to the invention impedes biological fouling, it seems likely that it is the result of the formation of hydroxyl ions on the surface S. These hydroxyl ions are formed as a result of the electrochemical decomposition of water.

Although the production of hydroxyl ions appears to be the most important in the method according to the invention, other (reaction) effects may (co-) determine the success of the method. This concerns, for example: hindrance of organisms by influencing the bio-electrochemistry of the organisms by the presence of charge on the surface to be protected against fouling or by varying the charge on this surface over time; - hindrance (physical hindrance) of organisms due to the formation of hydrogen gas on the surface to be protected against fouling; 10174 1 p 4 hindrance of organisms due to oxygen depletion on the surface to be protected against fouling (after all, the oxygen is consumed in the oxygen reduction reaction); hindrance of organisms because no biofilm, or only to a limited extent, can be formed.

With regard to fouling, the following organisms are of particular interest: algae, diatoms, pipe pipes, polyps, anemones, moss creatures, tube worms, clams and crustaceans, in particular barnacles, mussels, algae and lime duct worms (English / Latin names: Algae, diatoms , Ascidians, Hydroids, Anthozoa, Bryozoa, Tubeworms, Bivalve molluscs and Crustacea, especially barnacles, mussels, algae and tubercules).

According to the invention, settling of microorganisms such as bacteria and the formation of their biofilm (a very thin "slime film" formed by microorganisms) can also be prevented or controlled. This has the following beneficial effects: microbiological corrosion (MIC) is prevented or the risk of MIC occurring is reduced; fouling (macro fouling) is impeded because no biofilm is present.

The surfaces (S) that can be protected according to the invention are, for example: a) inner walls of systems through which (M) are passed such as cooling system pipes, heat exchangers or liquid transport pipes and associated parts which come into contact with (M); b) interior walls and parts of installations, equipment or storage facilities in which 25 (M) is subjected to certain treatments, such as filter installations, purification installations or reaction vessels.

c) outer walls of vessels and structures that are in contact with (M).

The surface (S) to be protected consists of or preferably contains: a) iron, copper, nickel, titanium, aluminum, or an alloy based on these metals, b) electrically conductive or semi-conductive coating or cover layer, such as a metallic cover layer , ceramic coating, intrinsically conductive polymer or paint system to which electrically conductive components have been added, .1017412 5 c) non-metallic non-conductive construction material or coating to which a conductive component, for example in the form of a filler or fibers, has been added, d) non-metallic, conductive or semi-conductive construction material. The best results are obtained with the method according to the invention if S contains steel, and especially if S contains stainless steel.

It has surprisingly been found that the method according to the invention not only protects against biological fouling, but also prevents corrosion of metals present in S which do not belong to the group of noble metals. This effect is even observed under conditions where for a long time the maximum potential assumes a value above the potential that would normally be used to achieve cathodic protection.

In order to properly polarize the surface to be protected - a relatively low potential must be applied - the surface in contact with water must of course have such a conductivity that the negative potential can be applied to it. The method according to the invention will therefore not work with the use of electrically insulating paint systems. In general it can be stated that the electrical conductivity of (S) must be such that a potential reduction of at least 300 mV, preferably of at least 500 mV, can be achieved within one minute.

In the method according to the invention, it is preferable that the maximum value of P applied by polarization is lower than the corrosion potential of S in M, since conditions are thus prevented that can lead to corrosion of S, especially in those cases where S contains one or more metals that do not belong to the noble metal group. The maximum value of P is preferably at least 50 mV lower, more in particular at least 100 mV lower than the corrosion potential.

Furthermore, it is advisable to keep the range of potential fluctuations within certain limits. Preferably, the fluctuations of P with respect to SCE are in the range of -300 to -3000 mV, and preferably -400 to -30 2000mV. Particularly good results have been obtained with stainless steel if the fluctuations mentioned are within the range of -800 to -1800 mV. The amplitude of the potential fluctuations is preferably at least 50 mV, and more in particular at least 100 mV. The frequency of the fluctuations is preferably at least once every 24 hours. Particularly preferred is a method in which the amplitude is at least 200 mV and the frequency at least once every 6 hours.

The desired biological inhibition can be achieved by applying (P) 5 (T) to (S), or varying (P) in any way over time. All kinds of conceivable variations of (P) over time are possible, but in general the method according to the invention will use pulse patterns (application of so-called spikes), wherein the periods of applying (P) to (S) a duration of 0.01-600 seconds, preferably 5-120 seconds, and (T) is 0.1 seconds - 48 hours, preferably 10 seconds - 4 hours.

In a particular embodiment, the invention relates to the use of a negative, time-fluctuating potential (P) for protecting electrically conductive surfaces (S) that are in contact with or come into contact with an aqueous medium (M) with the characterized in that P both a) hinders the growth of organisms that live in and / or reproduce in M and that tend to form deposits on S, and b) prevents corrosion of S by means of cathodic protection.

With active cathodic protection, the corrosion rate of a metal or metal alloy such as steel is greatly reduced with a rectifier by continuously lowering the potential to approximately -800 to -1000 mV compared to SCE. Surprisingly, by ensuring that in the application according to the invention P does not exceed the potential required for obtaining cathodic protection, two objectives can be achieved simultaneously. On the one hand biological fouling is prevented, on the other hand it is prevented that the surface to be protected is affected by corrosion.

Finally, the present invention also relates to a device for protecting against biological fouling of outer walls of vessels and / or other constructions that normally come into contact with water for a long period of time, as well as of inner walls of systems through which water is passed, which device comprises : a) an electrically conductive inner or outer wall, b) a voltage source connected to the inner or outer wall, which can realize a potential reduction of at least 300 mV on the wall within 1 minute, characterized in that the voltage source is capable of realizing a time-varying negative polarization on the surface to be protected from fouling. With this polarization, a counter electrode is normally used, which is positively polarized by the voltage source. This counter electrode is electrically isolated from (S) and preferably consists (but not necessarily) of a material that is inert in the medium M. The control of the voltage source is preferably effected with the aid of a reference (S) to be placed close to (S). electrode.

The invention is further illustrated in the following examples.

10

Examples

The experiments described below used larvae of the barnacle Balanus amphitrite. These larvae were grown on a laboratory scale, and the effect of the method according to the invention on the so-called settlement behavior of smallpox was investigated.

For the experiments, a test cell was used in which the settlement behavior of smallpox larvae can be investigated on a substrate whose potential can be electrochemically controlled. This test cell consisted of a plexiglass tube that was glued to a test plate. In the plexiglass "inner cell" that is created, smallpox larvae were introduced and fresh seawater was continuously supplied. Outflow of the seawater took place via a very fine filter mesh, so that the smallpox larvae could not leave the inner cell.

The test plate was connected via a time switch to a potentiostat, while a reference electrode was provided in the inner cell. On the outer wall of the Plexiglas cell, in the so-called "outer cell", a platinum counter-electrode was applied. With this the test plate could be polarized, without the reaction products of the counter-electrode being able to influence the smallpox larvae.

30

This test method can be used to determine whether smallpox larvae are able to settle on a test plate, or if they die due to a lack of a suitable location for settlement, or whether they settle down at a location other than the test plate.

1017412 8

The test conditions used were as follows: • Environment: natural seawater, with a refresh rate of the inner cell of approximately lx per hour

5 · Temperature: 26 ° C

• Test duration: 2 days • Approx. 200 - 300 larvae added per measuring cell • The larvae have not been fed in the meantime 10 The data regarding the applied potential pulses are given in Table 1. During the exhibition, the maximum output of the potentiostats was used to generate the pulses. The current potential level of the pulses was measured during the tests and differs slightly per test cell.

15

Table 1. The settings used per test cell I cell cell 2 cell 3 cell 4 cell 5 Ref 1 *> I Ref2 Ref3 (cell 5) pulse frequency H hour H hour 1 hour 1 hour H hour H hour No no pulse duration 2 min. 2 min. 2 min. ^ min. 2 min. 2 min. None none

Potential level -1.55 V -1.58 V -1.37 V -1.46 V -1.57 V -1.58 V EC01T Ecorr *) = Stainless steel in outer cell, to influence the development of exclusively chlorine gas on the larvae to observe.

20

Three reference cells were used during the tests. Reference cell 1 is used to ensure that the reaction products that arise on the platinum auxiliary electrode in the outer cell (such as chlorine gas) do not affect smallpox in the inner cell. To this end, in Reference cell 1, instead of the test piece in the inner cell, a strip of stainless steel is polarized, which is arranged around the plexiglass (in the outer cell). The reaction products on the auxiliary electrode are therefore the same as with the test cells 1, 2, 3,4, and 5, but there is no question of polarization of the test piece.

In Reference cells 2 and 3, smallpox larvae have been done without any potential control. If the larvae are in good condition, a problem-free settlement must also occur here 1017412 9. Reference cell 2 is identical to the cells discussed above, while reference cell 3 consists of a beaker.

The entire experiment lasted 2 days. At the start a fairly large quantity of larvae was used, which, partly in view of their mobility, were of good quality.

The results of the settlement tests are shown in Table 2. It appears here that growth is prevented in all test cells in which potential pulses have been applied. This while in all three reference cells there is strong growth.

Table 2: Results of the settlement tests

Cell No. Number of larvae Temp. Inspection on 01.12.1999 v. W. b. settlement: disabled

No larvae settled on the test plate 1. ca. 200-250 ca. 26 ° C However, strong adhesion of lime.

About 50 larvae settled on the wall.

No larvae settled on the test plate 2. ca. 200 ca. 26 ° C However, strong adhesion of lime.

A few larvae settled on the wall.

No larvae settled on the test plate 3 · ca-250 ca · 2 (> 0C Adhesion of lime, with cavities in the lime layer.

Larvae settled on the wall.

No larvae settled on the test plate 4 · ca 250 ca · 26 ° c Low adhesion of lime on a part of the test plate. Other parts of the test plate are bare. Larvae settled on the wall.

No larvae settled on the test plate 5. approx. 250 - 300 approx. 26 ° C Strong adhesion of lime.

Large number of larvae settled on the wall.

Ca. 40 settled on the test plate. These are very active Ref. 1 approx. 250 approx. 26 ° C (mobile).

The same number is still settling down on trial. Further settlement on the wall ..

Ca. 50 settled on the test plate.

Ref. 2 ca. 250 - 300 ca. 30 ° C Some free larvae that still have to settle down.

1017412 10

Many larvae on the andelteltled Glass surface completely filled with settled Ref. 3 remains of the approximately 30 ° C cyprid larvae. population 1017412

Claims (10)

A method for protecting against biological fouling of surfaces 5 (S) that are in contact with or come into contact with an aqueous medium (M) wherein 1. S is electrically conductive and 2. a potential (P) fluctuating in time is applied to S in such a way that it impedes the growth of organisms that live in M and / or reproduce in M and that tend to form deposits on S, characterized in that P does not assume values higher than the corrosion potential of S in M and the average value of P is lower than the same corrosion potential. 2. Method according to claim 1, characterized in that the maximum value of P is at least 50 mV lower than the corrosion potential of S in M. 3. Method 15 according to claim 1 or 2, characterized in that fluctuations of P with respect to SCE are within the range of -300 to -3000 mV. Method according to one or more of the preceding claims, characterized in that the amplitude in the potential fluctuations is at least 50 mV and the frequency is at least once every 24 hours. A method according to claim 4, characterized in that P is applied to S at intervals (T) or varies in some way over time and the periods of application of P to S have a duration of 0.01-600 seconds and T is 0.1 seconds - 48 hours. 6. Method as claimed in one or more of the foregoing claims, characterized in that the electrically conductive capacity of S is at least such that within 1 minute at S a potential reduction of at least 300 mV can be realized. 7. Method as claimed in one or more of the foregoing claims, characterized in that S comprises: a) inner walls of systems through which M is passed, such as cooling system pipes, heat exchangers or liquid transport pipes and associated parts which come into contact with M; (b) internal walls and parts of installations, equipment or storage facilities in which (M) is subjected to certain treatments, such as filter installations, purification installations or reaction vessels. (c) external walls of vessels and structures in contact with Μ. Method according to one or more of the preceding claims, characterized in that S consists of or contains: a) iron, copper, nickel, titanium, aluminum, or an alloy based on these metals, b) electrically conductive or semiconductive coating or cover layer such as metallic cover layer, ceramic cover layer, intrinsically conductive polymer or paint system to which electrically conductive components have been added, c) non-metallic non-conductive construction material or cover layer to which a conductive component, for example in the form of a filler or fibers, has been added , d) non-metallic, conductive or semiconductive construction material. 9. Use of a negative time-fluctuating potential (P) for protecting electrically conductive surfaces (S) that are in contact with or come into contact with a water-containing medium (M), characterized in that both P a) inhibits the growth of organisms that live in and / or reproduce in M and that tend to form deposits on S, as well as b) prevents corrosion of S by means of cathodic protection. 10. Device for protecting against biological fouling of the outer walls of vessels and / or other structures that normally come into contact with water for a long period of time, as well as the inner walls of systems through which water is passed, which device comprises: a) an electrically conductive inner or outer wall, b) a voltage source which is connected to the inner or outer wall and which can realize a potential reduction of at least 300 mV on the wall within 1 minute, characterized in that the voltage source is capable of a time-varying achieve negative polarization on the surface to be protected against fouling. 1017412